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Oceanography Brown Bag Seminar
Mechanism of Gulf Stream
Separation and the Implication for Low Frequency Variability in the
North Atlantic
Wednesday, March
17, 2004
12:00 noon – 1:00 p.m.
NCAR Mesa Lab, Chapman Room
Abstract:
The mechanism of the Gulf Stream separation and the formation of the
cyclonic northern recirculation gyre (NRG) are investigated using a
hierarchy of models. Modeling results show that the Gulf Stream path and
the formation of the NRG are sensitive to both the magnitude of lateral
viscosity and the strength of deep western boundary current (DWBC). In
particular, the bottom vortex stretching induced by the downslope DWBC
near south of Grand Banks leads to the formation of the NRG and keeps the
Gulf Stream path in the open ocean separated from the North American
coast. Analytical solutions show that the bottom vortex stretching is
important near the western boundary only when the continental slope is
wider than the Munk frictional layer scale.
The above mechanism indicates that on low frequency
the variations of the NRG and Gulf Stream path can be modulated by the
variations of DWBC. Modeling results show that the Great Salinity Anomaly
(GSA) events play an important role in causing large scale low frequency
variability in the North Atlantic. The GSA events lead to significant
weakening of the thermohaline circulation and DWBC, and thus weakening of
the NRG and northward shift of the Gulf Stream path on decadal timescales,
resulting in a dipole pattern (warming off US east coast and cooling south
of Greenland). Diagnoses of the 20th century observations show that on
long timescales, the positive (negative) GSA phase, associated with more
(less) Iceland sea ice extent, leads Labrador Sea surface cooling
(warming) and a positive (negative) North Atlantic Oscillation (NAO)
phase by 3 and 7 years respectively. A physical mechanism for the phase
relationship is discussed.
For more information, contact: Lisa
Butler
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